The present invention relates to a method for producing a silicon wafer wherein there is no mechanical damage, surface roughness is small and there is no slip dislocation.
Recently, a silicon single crystal wafer is mainly used as a wafer for fabricating a device such as a semiconductor integrated circuit or the like. In the device such as a semiconductor integrated circuit or the like, a design rule of the device recently tends to get smaller. Accordingly, some problems that previously had not mattered so much have been getting serious and important. Examples of those problems include mechanical damages, scratches and micro roughness introduced in a step of processing a wafer. It has been getting more important especially nowadays to remove the mechanical damages or the like, to provide a wafer having good flatness.
Because, the mechanical damages, scratches and micro roughness will be serious problems in a process for fabricating a device, especially in a process for fabricating a recent device having a small design rule and being highly integrated. For example, the mechanical damages, scratches or the like on the surface of the wafer cause local diffusion abnormality in a process of diffusion or ion plantation that is a process for incorporating impurities, and cause local oxidation abnormality in an oxidation process. They cause life time of the wafer to be shorter. Such a harmful influence due to mechanical damages or the like gets larger in higher integrated device.
Conventionally, in manufacture of a semiconductor silicon wafer, crystal defects such as mechanical damages or the like have been introduced in the wafer in various processes of processing a silicon single crystal to be a wafer, and the wafer has been used in a process for fabricating a device, without removing the mechanical damages or the like. Such mechanical damages or the like are often introduced, especially in a polishing process.
Namely, a polishing process generally conducted in production of a silicon wafer is a multi-step polishing process comprising a combination of chemical etching and mechanical polishing. Such a multi-step polishing process comprises, for example, processes called in order first polishing, second polishing, (third polishing, if desired), final polishing. Polishing conditions are varied in each of the steps, for example, abrasive grains having smaller grain size or a polishing pad having lower hardness is used in the later step.
In that case, since higher flatness is required in the surface of the wafer finally obtained, it was necessary to conduct polishing wherein mechanical character is enhanced (mechanically enhanced polishing) at least in the final step of the polishing process (final polishing) to make micro roughness of the finished surface small. Accordingly, the wafer is stressed much by the mechanically enhanced polishing, so that scratches and slight mechanical damages may remain on the surface. If polishing wherein chemical character is enhanced (chemically enhanced polishing) such as a second polishing is mainly conducted, almost no mechanical damages remain, but flatness of the surface is degraded resulting in large micro roughness.
Such problems cannot be avoided, in the case that mechanical polishing and chemical etching are combined in a polishing process. It is impossible to make the micro roughness small enough and prevent introduction of mechanical damages in the processing method according to conventional polishing methods.
In order to overcome disadvantages of the conventional methods for polishing a wafer, there has been disclosed in Japanese Patent Application Laid-open (Kokai) No.7-235534, a technique for substituting heat treatment in a hydrogen gas atmosphere for the final polishing step in a method for producing a silicon wafer, which uses a phenomena that the surface of the silicon wafer is etched by heat treatment in a hydrogen gas atmosphere.
In such a technology, the wafer obtained in the polishing step before the final polishing is subjected to the heat treatment in a hydrogen atmosphere without conducting a final polishing, and the same degree of surface roughness as that may be obtained in the final polishing can be obtained by the above-mentioned etching effect. Such a method seems to have an effect of enabling high flatness of the surface without introducing the mechanical damages caused by the mechanically enhanced polishing, since the intended surface roughness can be achieved even when the final polishing process is omitted. In such a method, it is necessary to conduct the heat treatment in a hydrogen atmosphere at a temperature of 1200xc2x0 C. or more for a period from 30 minutes to four hours.
According to the above-mentioned method, removal of mechanical damages and scratches, and lowering of micro roughness are possibly achieved at the same time. However, there is a disadvantage that slip dislocation is easily generated in the wafer during the heat treatment in the method. Such a problem is serious, especially when a wafer having a large diameter is subjected to the heat treatment. Since a wafer has been getting larger recently, such a disadvantage is a serious problem. Furthermore, since an etching effect on the surface is used in the above method, there is a problem that thickness of the wafer is inevitably changed before and after the heat treatment.
The heat treatment at high temperature of 1200xc2x0 C. or more for a long time has also a disadvantage that productivity is lowered, since it also needs a long time for raising and lowering temperature, as a result, longer time is required for the whole heat treatment process.
As described above, there is no conventional method for producing a silicon wafer wherein mechanical damages are not introduced on the surface of the wafer, micro roughness is sufficiently controlled to improve surface roughness, and slip dislocation is not introduced, and productivity is high. Therefore, development of a substitute for the conventional methods has been required.
The present invention has been accomplished to solve the above-mentioned problems. An object of the present invention is to provide a method for producing a silicon wafer that can produce in high productivity a silicon wafer wherein there is neither mechanical damages nor scratches on the surface of the wafer, surface roughness is significantly improved, and there is no slip dislocation.
To achieve the above mentioned object, the present invention provides a method for producing a silicon wafer wherein at least one surface of the wafer is subjected to a multi-step polishing process, in which a heat treatment in a mixed gas atmosphere of hydrogen and argon through use of a rapid heating/rapid cooling apparatus is substituted for a final polishing in the multi-step polishing process.
As described above, if the heat treatment in a mixed gas atmosphere of hydrogen and argon through use of a rapid heating/rapid cooling apparatus is substituted for a final polishing, mechanically enhanced polishing as a final polishing step can be omitted, so that there can be provided a wafer wherein mechanical damages are not introduced on the surface of the wafer, and surface roughness is sufficiently improved by the rapid thermal annealing. Furthermore, since the wafer is subjected to the heat treatment in a mixed gas atmosphere of hydrogen and argon, slip dislocation can also be prevented from generating in the wafer. Since the heat treatment is conducted through use of a rapid heating/rapid cooling apparatus, the heat treatment is not necessary to be conducted for a long time, so that a silicon wafer can be produced in high productivity.
In that case, a ratio of hydrogen gas to a mixed gas of hydrogen and argon is preferably 20 to 40% by volume.
Because, if a ratio of hydrogen gas to a mixed gas of hydrogen and argon as an atmosphere for the heat treatment is 20 to 40% by volume, especially, generation of slip dislocation can be prevented almost completely.
The above-mentioned heat treatment is preferably conducted at a temperature of 1100 to 1300xc2x0 C. for 1 to 60 seconds.
Because, if the heat treatment is conducted at a temperature of 1100xc2x0 C. or more, surface roughness on the surface of the wafer can be improved more effectively. If the heat treatment is conducted at a temperature of 1300xc2x0 C. or less, generation of slip dislocation can be prevented more effectively. If the heat treatment is conducted for one second or more, surface roughness on the surface of the wafer can be improved effectively, and 60 seconds will be enough to obtain such an effect. If the heat treatment is conducted for 60 seconds or less, the silicon wafer can be produced in significantly high productivity.
Furthermore, the silicon wafer produced according to the method of the present invention is, for example, a silicon wafer wherein mechanical damage is 12.5 pm (pico meter) or less as PAD value, the surface roughness is 1.0 nm or less as Pxe2x88x92V value (the maximum difference between peek and valley) in 2 by 2 xcexcm square measured with an atomic force microscope, and there is no slip dislocation.
As described above, the silicon wafer of the present invention is the wafer containing substantially no mechanical damages, and having excellent surface roughness and having no slip dislocation. Accordingly, it can be used for a semiconductor device or the like that will have a small design rule and be highly integrated in future.
PAD (Photo-Acoustic Displacement) value herein means an amount of variation of photothermal effect resulting from increase of diffusion temperature due to heat converted from light absorbed on the surface when the surface of the wafer is exposed to exciting light. Evaluation of the value enables accurate evaluation of mechanical damages of the wafer.
As described above, according to the method for producing a silicon wafer of the present invention, there can be produced in high productivity the wafer wherein there is no mechanical damages nor scratches on the surface of the wafer, the surface roughness is extremely improved, there is no slip dislocation. Subsidiary effects of the present invention are that the process can be simplified by omitting the polishing step to be finally conducted, and that COPs can be reduced.